Theresa May is facing calls to abandon her Brexit "red lines" after aerospace giant Airbus warned it could pull out of the UK with the loss of thousands of jobs if Britain crashes out of the EU without a deal.

The company - which employs 14,000 people at 25 sites across the country and has a presence in Scotland at Prestwick - said it would "reconsider its investments in the UK, and its long-term footprint in the country" if Britain was forced to leave the single market and customs union in March 2019 without any transition agreement in place.

"The Government's reckless decision to keep no deal on the table and to rule out a customs union or strong single market deal after Brexit is putting jobs and the economy at risk," he said.

"Ministers need to start listening to legitimate concerns of businesses and get a grip of the Brexit negotiations."

Unite's assistant general secretary Steve Turner said it was imperative the Government avoided a "cliff edge" Brexit.

"It would be a betrayal of Airbus workers, their families and the tens of thousands of workers in the wider supply chain if the Government failed to secure frictionless trade and access to the customs union and single market," he said.

"Twenty years ago I heard car manufacturers saying if Britain didn't join the euro they may well consider pulling out of Britain - Nissan, others like that," he told Sky News.

"Big business will always lobby for their interests, of course they will."

In a Brexit "risk assessment" published on its website, Airbus called on the Government to extend the planned transition period due to run until December 2020, saying it was too short for the business to reorganise its supply chain.

Tom Williams, the chief operating officer of Airbus Commercial Aircraft, said Brexit would have "severe negative consequences" for the UK aerospace industry whether or not there was an agreement with Brussels.

"While Airbus understands that the political process must go on, as a responsible business we require immediate details on the pragmatic steps that should be taken to operate competitively, he said.

"Without these, Airbus believes that the impacts on our UK operations could be significant.

"We have sought to highlight our concerns over the past 12 months, without success. Far from Project Fear, this is a dawning reality for Airbus.

A Government spokeswoman said that while officials were working closely with companies to understand their concerns, they did not expect a "no deal" scenario to arise.

"We have made significant progress towards agreeing a deep and special partnership with the EU to ensure trade remains as free and frictionless as possible, including in the aerospace sector, and we're confident of getting a good deal that is mutually beneficial," the spokeswoman said.

Downing Street denied that ministers had ignored concerns raised by Airbus about the Brexit negotiations.

"Airbus were in Downing Street with the PM in April as part of an aerospace round table," a No 10 spokeswoman said.